In prior processes for obtaining concentrates by freeze concentration, up to 50% of the contained water could be easily eliminated. It has been commercially difficult to eliminate the additional water.
The exceedingly high concentration of sugar and other substances always produced increasingly smaller crystals (microcrystals) which are difficult to separate in a satisfactory manner. Where it was economic to separate such micro-crystals, they carried down, by absorption, much of the raw initial feedstock. The resultant losses made it uneconomical to concentrate the product beyond the 50% concentration.
The viscosity of the concentrate also continuously increased until the mother liquors could no longer be economically and/or conveniently pumped.
To avoid either or both of these problems, most installations operate in two separate unit operations i.e. they concentrate the initial material by a freeze-crystallization process and then they subject this concentrate to other concentration means i.e. high vacuum evaporation by the thin wall method, spray drying, cryo-drying and even high-surface area diffusion processes. These subsequent unit operations utilize much energy, require highly sophisticated equipment and, unless the final products are extremely valuable (drugs), are generally uneconomical.
In most of these secondary-concentration procedures, the feedstocks are heated to some extent in order to evaporate the residual water. This heating step results in some loss of aroma and the vitamins are often destroyed by oxidation. In fact the oxidation of other labile components, as in fruit juices, cannot be avoided.
As a result, several varied systems have been employed for the final concentration operation which aim at the recovery of the lost aroma. In general these have not been completely effective.
It has been an object of this invention to find a process which is not subject to the above mentioned disadvantages.
Many crystallization methods are directed at removing the more readily crystallizable fractions by cooling and thus concentrating the other components and/or even purifying them through the elimination of the crystallizable diluents or impurities. Many of the known methods are based upon an indirect cooling of the liquid in suitable containers. After the desired low temperature has been obtained and the crystallization of the more readily crystallizable components is achieved, the next step consists in the separation of the crystals from the mother liquor by standard means such as filter presses, centrifuges, screw-expellers and other suitable liquid-solid separating apparatus.
In these known methods, the crystals are separated from the liquid in scraper-coolers, with indirect contact between the liquid and the refrigerant or coolant via the cooled surfaces. The crystals form on the wall that is at a low temperature and hence the crystal adheres thereto and must be scraped off. These methods have three primary disadvantages:
(a) the crystals are comminuted during removal;
(b) impurities arise as a result of the mechanical wear of the scraper and the scraped surfaces;
(c) interruptions in the process often occur due to clogging of the separation apparatus by the comminuted crystals.
New processes based upon direct cooling, where the refrigerant is contacted with the feedstock in a co-current or counter-current manner, have not as yet been economically practical. These newer processes generally require that the specific gravities of the refrigerant, the feedstock and the crystals must differ in order to be able to achieve a suitable separation in the subsequent separations steps. Furthermore, they require that the refrigerating fluid be imiscible with the crystallizable substances. To achieve good separation of the crystals, substantial equipment investment is required and such operations must be carried out with great care in order to produce crystals of proper size to avoid difficulties in separation.